In normal operation, an aircraft which is equipped with several engines is propelled by the generation of a symmetrical thrust over its various engines, that is to say a thrust which is such that the resultant moment of the propulsion forces about a vertical axis (with respect to the aircraft) is zero. Thus, a maneuvering capability in yaw (about the vertical axis) which is identical to the left and to the right is guaranteed, the yaw control being implemented by deflecting the rudder of the aircraft.
Moreover, the orientation of the aircraft's path in the horizontal plane is carried out by the inclination of the lift. This inclination is achieved more or less rapidly and necessitates a contribution from the rudder in order to guarantee a low or even zero sideslip. The speed of inclination is a determinant parameter for the deflection amplitude necessary for the rudder.
In the case of a failure or reduction of the thrust of one or more engines of the aircraft, the aforesaid condition of symmetry of the thrust of the engines is no longer achieved. Thus, in order to maintain symmetrical flight, it is then necessary to implement a rudder deflection which compensates for the yaw moment of the propulsion forces which, when the aircraft is put into a turn, is added to the deflection intended for maintaining zero sideslip during this maneuver. The consequence of this is:                either to cause to a large increase in the aerodynamic loads undergone by the structure of the rear parts (tail, vertical stabilizer, etc.) of the aircraft during this maneuver;        or, if the rudder stop is reached before the yaw requirement is fully met, to cause a high degree of sideslip, which reduces the safety margins and also causes a high structural load level due to sideslip.        
From the document FR-2 753 171, there is known a device for controlling the thrust of a multi-engined aircraft during a thrust asymmetry due to the failure of an engine of the aircraft.
Moreover, from the document FR-2 879 561, there is known a particular method for improving the roll piloting of an aircraft, this method in particular using means for generating a roll command.